1. Field of the Invention
This invention relates to a heavy duty pneumatic tire, and more particularly a pneumatic radial tire having an improved tread for trucks, buses and the like.
2. Description of the Prior Art
Characteristics of tires required for heavy duty vehicles such as trucks and buses are, first of all, the wear resistance from the economical point of view, and second, the antiskid characteristic on wet paved roads from the viewpoint of safety.
It has been known that the wear resistance depends generally on the kinds of compounds used for the treads and on the design of the treads, particularly, the configuration and arrangement of tread patterns figures no less greatly than the kinds of compounds. On the other hand, the antiskid characteristic on a wet road depends absolutely on the design factors of the treads. The antiskid characteristic can be generally improved by increasing the percentage of grooves occupied in a tread to aid the drainage in a contact surface.
As can be seen from this explanation, the wear resistance and antiskid characteristic are contradictory. Accordingly, one of these required characteristics of tires has been unavoidably sacrificed more or less according to the application of the tires.
There have been known two general tread types, one of which is the so-called five rib type wherein four wide zigzag circumferentially endlessly continuous grooves are arranged equally spaced in the direction of the width of the tire, and the other of which is the three rib type wherein a pair of wide zigzag circumferentially continuous grooves are arranged to divide the width of the tread into three parts and the divided ribs are formed with grooves which are so narrow that they are closed when the tire is loaded.
The five rib type tread is superior in antiskid characteristic but it is inferior in wear resistance. The three rib type tread is rather inferior in antiskid characteristic. These two types thus have one merit and one demerit contradictory to each other.